Now that savings in fuel and the need to protect the environment have become a priority, it is desirable to produce mixtures having good mechanical properties and a hysteresis which is as low as possible in order to be able to employ them in the form of rubber compositions that can be used in the manufacture of various semi-finished products incorporated into the composition of pneumatic tires, such as, for example, undertreads, side walls, treads, and in order to obtain tires having a reduced rolling resistance.
The reduction of the hysteresis of the mixtures is a permanent objective which must however be carried out while retaining the processability of the mixtures.
In order to achieve the objective of a drop in hysteresis, numerous solutions have already been tested. In particular, mention may be made of the modification of the structure of the diene polymers and copolymers at the end of polymerization by means of functionalizing, coupling or star-branching agents for the purpose of obtaining a good interaction between the polymer thus modified and the filler, whether it be carbon black or a reinforcing inorganic filler.
Within the context of a mixture containing a reinforcing inorganic filler, it has been proposed to use functionalized diene polymers in place of the unfunctionalized polymers which were used previously, and in particular polymers functionalized by alkoxysilane derivatives.
By way of illustration of this prior art relating to reinforcing inorganic fillers, mention may, for example, be made of the U.S. Pat. No. 5,066,721, which describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolysable alkoxy residue as a mixture with silica.
One drawback of these reactions for functionalization by an alkoxysilane derivative lies in the subsequent implementation of the steam-stripping operation, which is necessary in order to remove the polymerization solvent.
Indeed, generally, experimentation shows that the functionalized polymers obtained undergo changes in the macrostructure during this stripping operation, which leads to a severe degradation of their properties, unless they are limited to using, as a functionalizing agent, an alkoxysilane belonging to a restricted family, such as that which is described in the aforementioned document U.S. Pat. No. 5,066,721.
This is the reason why research has been carried out on other functionalization reactions, still with a view to obtaining such rubber compositions.
By way of example, mention may be made of the patent EP 0 778 311 B1 in the name of the applicant, which discloses the incorporation, into rubber compositions comprising, as a reinforcing filler, predominantly silica, of diene polymers bearing, as a chain end, a silanol functional group or a polysiloxane block having a silanol end. Use is made, for example, of a functionalizing agent constituted of a cyclic polysiloxane, such as hexamethylcyclotrisiloxane. The functionalized polymers obtained may be separated from the reaction medium resulting in their formation by steam extraction of the solvent, without their macrostructure and, consequently, their physical properties changing.
It has been possible to establish that these polymers confer rubbery, especially hysteresis, properties and reinforcing properties in the crosslinked state, which are improved relative to those of control compositions based on unfunctionalized diene polymers, and which are at least analogous to those of compositions based on diene polymers comprising an alkoxysilane functional group.
However, these polymers, which comprise an active silanol functional group for coupling to silica or to carbon black which is surface-modified with silica, although giving the rubber compositions incorporating them an improvement of the hysteresis and reinforcing properties, generally result in a processability of the uncrosslinked mixtures which is impaired relative to that of unfunctionalized “control” polymers.
Other means of reducing the hysteresis have therefore been sought which allow a better processing of the mixtures.
Patent EP 1 127 909 B1 in the name of the applicant discloses a process for preparing, and the use in a vulcanizable rubber composition, of a copolymer having a polyether block at the end of the chain. This copolymer is intended to interact with the reinforcing inorganic filler so as to reduce the hysteresis of the mixture. The process for preparing this copolymer comprises a complex method of grafting the polyether block in three steps: the functionalization of the ends of living polymer chains by a cyclic organosiloxane, the reaction of the living polymer thus functionalized with a dialkyl dihalo silane, then the reaction of this Si—X (X being a halogen) functionalized polymer with a polyethylene glycol in the presence of dimethyl amino pyridine. It appears that the hysteresis properties of the rubber composition comprising the copolymer are significantly improved relative to a composition comprising a non-functional elastomer, while enabling an improved processing relative to a composition comprising an elastomer functionalized at the chain end by a silanol functional group.
U.S. Pat. No. 6,518,369 proposes a reinforced rubber composition containing a diene copolymer having a polyether block, and also a process for preparing said copolymer. The solution used consists in reacting living chains of diene elastomers with a specific polyether. Although proposing an improvement in the degree of grafting of the polyether to the chains of polymer prepared in solution, this degree of grafting remains insufficient with the process described in this patent. The grafting efficiency of the polyether block is the deciding factor for the quality of the interaction of the block copolymer with the reinforcing filler in a reinforced rubber composition, and therefore for the mechanical properties of this composition.